Generally, thick film conductor paste compositions contain a conductive component, a binder, and an organic vehicle as the principal components. As the conductive component, fine powders of the noble metals palladium (Pd), platinum (Pt), and silver (Ag), or mixtures or alloys thereof, or oxides of palladium and silver, or mixtures thereof, have been widely used. Glass binder is used for the binder, and an inert liquid is used as the organic medium.
These thick film conductor pastes are applied to the substrate by screen printing and fired in the printing pattern. For the substrate, a ceramic substrate made from alumina or aluminum nitride is often used.
In recent years it has been desired to employ aluminum nitride substrates for circuit boards used under high-temperature environments because of their high heat conduction coefficient. However, in order to adhere metal conductors to aluminum nitride substrates, it was necessary to use thick film technology, which adheres the conductor via a thin reactive layer (oxide film) formed between the metal and substrate by introducing the metal in atomic form to the surface of the ceramic substrate so that the metal, which is extremely active chemically, bonds with the excess oxygen that exists in the surface of the substrate. The required large and expensive equipment of the pulsed laser accumulation method, etc., entailed such problems as poor reactions because of the nitrogen contained or the evolution of nitrogen oxides. Therefore, the development of thick film conductor paste compositions with which thick film technology can be applied to aluminum nitride substrates and using a conductor which can be formed as "thick film" on aluminum nitride substrates by printing or coating the conductor paste and subsequently firing has been desired.